plant 14 diseases caused viruses by
introduction
Viruses are entities that are too small to be seen with a light microscope, multiply only in living cells, and have the ability to cause disease. All viruses are parasitic in cells and cause a multitude of diseases to all forms of living organisms, from single-celled plants or animals to large trees and mammals. Some viruses attack man and/or animals and cause diseases such as influenza, polio, rabies, small pox, and warts,- others attack plants; and still others attack microorganisms, e.g., bacteria and myco- plasmas. The total number of viruses known to date is well over a thousand, and new viruses are described almost every month. More than half of all known viruses attack and cause diseases of plants. One virus may infect one or dozens of different species of plants, and one plant may be attacked by one or many different viruses. A plant may also commonly be infected by more than one virus at the same time.
Although viruses are agents of disease and share with other living organisms genetic functions and the ability to reproduce, they also be- have as chemical molecules. At their simplest, viruses consist of nucleic acid and protein, with the protein wrapped around the nucleic acid.
Although viruses can take any of several forms, they are mostly either rod shaped or polyhedral, or variants of these two basic structures. There is always only RNA or only DNA in each virus and, in most plant viruses, only one kind of protein. Some of the larger viruses, however, may have several different proteins, each probably having a different function.
Viruses do not divide and do not produce any kind of specialized reproductive structures such as spores, but they multiply by inducing host cells to form more virus. Viruses cause disease not by consuming
cells or killing them with toxins, but by upsetting the metabolism of the 549
cells which, in turn, leads to the development by the cell of abnormal substances and conditions injurious to the functions and the life of the cell or the organism.
characteristics of plant viruses
Plant viruses differ greatly from all other plant pathogens not only in size and shape, but also in the simplicity of their chemical constitution and physical structure, methods of infection, multiplication, translocation within the host, dissemination, and the symptoms they produce on the host. Because of their small size and transparency of their bodies, viruses cannot even be viewed and detected by the methods used for other pathogens. Viruses are not cells, nor do they consist of cells.
DETECTION
When a plant disease is caused by a virus, individual virus particles cannot be seen with the light microscope, although some virus- containing inclusions or crystals may be seen in virus-infected cells.
Examination of sections of cells or of crude sap from virus-infected plants under the electron microscope may or may not reveal viruslike particles.
Virus particles are not always easy to find under the electron microscope, and even in the rare cases in which such particles are revealed, proof that the particles are a virus, and that this virus causes the particular disease, requires much additional work and time.
A few plant symptoms, such as oak-leaf patterns on leaves and chloro- tic or necrotic ring spots, can be attributed to viruses with some degree of certainty. Most other symptoms caused by viruses resemble those caused by mutations, nutrient deficiencies or toxicities, insect secretions, by other pathogens, and other factors. The determination, therefore, that certain plant symptoms are caused by viruses involves the elimination of every other possible cause of the disease, and the transmission of the virus from diseased to healthy plants in a way that would exclude trans- mission of any of the other causal agents.
The present methods for detection of plant viruses involve primarily the transmission of the virus from a diseased to a healthy plant by budding, grafting, or by rubbing with plant sap. Certain other methods of transmission, such as by dodder or insect vectors, are also used to demon- strate the presence of a virus. Most of these methods, however, cannot distinguish whether the pathogen is a virus, mycoplasma or rickettsia- like bacterium; only transmission through plant sap is presently consid- ered as proof of the viral nature of the pathogen. The most definitive proof of the presence of a virus in a plant is provided by its purification, electron microscopy, and/or serology.
CHARACTERISTICS OF PLANT VIRUSES
MORPHOLOGY
Plant viruses come in different shapes and sizes, but they are usually described as elongate (rigid rods or flexuous threads), as rhabdoviruses (bacilluslike), and as spherical (isometric or polyhedral) (Figs. 202, 203, and 204).
Some elongated viruses like tobacco mosaic virus and barley stripe mosaic virus, have the shape of rigid rods with measurements about 15 x 300 nm and 20 x 130 nm, respectively. Most of the elongated viruses appear as long, thin, flexible threads that are usually 10 to 13 nm wide and range in length from 480 nm (potato virus X) to 2000 nm (tristeza virus). Many of the elongated viruses seem to occur in particles of differ- ing lengths, and the number given usually represents the length that is more common than any other.
The rhabdoviruses are short, bacilluslike rods, approximately three to five times as long as they are wide, as in the cases of potato yellow dwarf
FIGURE 202.
Electron micrographs of the various shapes of plant viruses. (A) Rod shaped (tobacco mosaic). (B) Flexuous thread (maize dwarf mosaic). (C) Isometric (cowpea chlorotic mottle). (D) Rhabdovirus (broccoli necrotic yellows). (Photo D from Lin and Campbell, Virology 4 8 : 3 0 - 4 0 , 1972.)
551
FIGURE 203.
Electron micrograph of alfalfa mosaic virus showing the various sizes of the five components of this virus, x 168,000. (Photo courtesy Ε. M. J. Jaspars, Univ. of Leiden, The Netherlands.)
virus which measures 75 x 380 nm, wheat striate mosaic virus (65 x 270 nm), and the lettuce necrotic yellows virus (52 x 300).
Most, and probably all, spherical viruses are actually polyhedral, rang
ing in diameter from about 17 nm (tobacco necrosis satellite virus) to 60 nm (wound tumor virus). Tomato spotted wilt virus seems to have a flexible, spherical shape 70 to 80 nm in diameter.
Many plant viruses consist of more than one component. Thus, to
bacco rattle virus consists of two rods, a long one measuring 195 x 25 nm and a shorter one varying in length from 43 to 110 x 25 nm; alfalfa mosaic virus consists of five components measuring 58 x 18, 54 x 18, 42
x 18, 30 x 18, and 18 x 18 nm (Fig. 203). Also, many isometric viruses have two or three different components of, usually, the same size but different weights as they contain different amounts of nucleic acid. In all the above cases more than one of the components must be present in the plant for the virus to multiply and perform in its usual manner.
The surface of both the elongated and the spherical viruses consists of a definite number of protein subunits, which are spirally arranged in the elongated viruses, and packed on the sides of the polyhedral particles of the spherical viruses (Fig. 204). In cross sections, the elongated viruses appear as hollow tubes with the protein subunits forming the outer coat and the nucleic acid, also spirally arranged, embedded between the inner ends of two successive spirals of the protein subunits. The spherical viruses may or may not be hollow, the visible shell consisting of the protein subunits, with the nucleic acid inside the shell and arranged in a yet unknown manner.
CHARACTERISTICS OF PLANT VIRUSES 553
FIGURE 204.
Relative shapes, sizes, and structures of some representative plant viruses. (A) An elongate virus appearing as a flexuous thread. (B) A rigid rod-shaped virus. (B-l) Side arrangement of protein subunits [PS) and nucleic acid (NA) in viruses A and Β. (B-2) Cross section view of the same viruses. HC = hollow core. (C) A short, bacillus-like virus. (C-l) Cross-section view of such a virus. (D) A polyhedral virus. (D-l) An icosahedron, representing the 20-sided symmetry of the protein subunits of the polyhedral virus.
T h e rhabdoviruses, p o t a t o y e l l o w dwarf virus, l e t t u c e n e c r o t i c yel
lows, e t c . are provided w i t h an o u t e r envelope or m e m b r a n e bearing surface projections. Inside t h e m e m b r a n e is t h e nucleocapsid, consisting of helically arranged n u c l e i c acid and a s s o c i a t e d protein subunits.
COMPOSITION AND STRUCTURE
E a c h plant virus consists of at least a n u c l e i c acid and a protein. S o m e viruses consist of m o r e t h a n o n e size of n u c l e i c acid and proteins, and s o m e of t h e m c o n t a i n additional c h e m i c a l c o m p o u n d s , s u c h as poly a m i n e s , lipids, or specific e n z y m e s .
The proportions of nucleic acid and protein vary with each virus, nucleic acid making up 5 to 40 percent of the virus and protein making up the remaining 60 to 95 percent. The lower nucleic acid and the higher protein percentages are found in the elongated viruses, while the spheri- cal viruses contain higher percentages of nucleic acid and lower percent- ages of proteins. The total weight of the nucleoprotein of different virus particles varies from 4.6 million molecular weight units (bromegrass mosaic virus) to 39 million (tobacco mosaic virus) to 73 million (tobacco rattle virus). The weight of the nucleic acid alone, however, ranges only between 1 and 3 million (1-3 x 106) molecular weight units per virus particle, compared to 0.5 x 109 for mycoplasmas, 1 x 109for spiroplasmas, and more than 1.5 x 109 for bacteria.
COMPOSITION AND
STRUCTURE OF VIRAL PROTEIN
Viral proteins, like all proteins, consist of amino acids. The sequence of amino acids within a protein is dictated by the genetic material, which in viruses is either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), and determines the nature of the protein.
The protein components of plant viruses are composed of repeating subunits. The amino acid content and sequence is constant for the identi- cal protein subunits of a virus, but may vary for different viruses, different strains of the same virus, and even for different proteins of the same virus particle. The content and partial sequences of amino acids are known for the proteins of several viruses, but only for the protein of tobacco mosaic virus (TMV) and of turnip yellow mosaic virus (TYMV) is the complete sequence of amino acids known. Thus, the protein subunit of TMV consists of 158 amino acids in a constant sequence. Similarly, the protein subunit of TYMV has 189 amino acids.
In TMV the protein subunits are arranged in a helix containing I 6 V 3 subunits per turn (or 49 subunits per three turns). The central hole of the virus particle down the axis has a diameter of 40 A, while the maximum diameter of the particle is 180 A. Each TMV particle consists of approxi- mately 130 helix turns of protein subunits. The nucleic acid is packed tightly between the helices of protein subunits. In the rhabdoviruses the helical nucleoproteins are enveloped in a membrane.
In the polyhedral plant viruses the protein subunits are tightly packed in arrangements that produce 20, or some multiple of 20, facets and form a shell. Within this shell the nucleic acid is folded or otherwise or- ganized.
COMPOSITION AND STRUCTURE OF VIRAL NUCLEIC ACID
The nucleic acid of most plant viruses consists of RNA. To date only three plant viruses (cauliflower mosaic, dahlia mosaic, and carnation etched ring) have been shown to contain DNA. Both RNA and DNA are long, chainlike molecules consisting of hundreds or, more often, thousands of units called nucleotides. Each nucleotide consists of a ring
CHARACTERISTICS OF PLANT VIRUSES 555 compound called the base attached to a 5-carbon sugar [ribose (I) in RNA,
deoxyribose (II) in DNA], which in turn is attached to phosphoric acid.
The sugar of one nucleotide reacts with the phosphate of another nuc
leotide, and this is repeated many times, thus forming the RNA or DNA strand. In viral RNA, one of only four bases can be attached to each ribose molecule. These bases are adenine, guanine, cytosine, and uracil. The first two, adenine and guanine, are purines, while cytosine and uracil are pyrimidines. The chemical formulas of the bases and one of their possible relative positions in the RNA chain, are shown in structure (III). DNA is similar to RNA with two small, but very important differences: the oxygen of the sugar hydroxyl is missing; and the base uracil is replaced by the base methyluracil, better known as thymine (IV).
The sequence and the frequency of the bases on the RNA strand vary from one RNA to another, but they are fixed within a given RNA and
OH
(i)
HOCH
(Π)
NH2
HC I
CH9
\ XJ. ^CH II
VN ^
HC II HC,
Adenine
NH I
OH ) — P — O - C H
// \
Ο ο
I
> — p -
ο ο
OH 0- C H2
Cytosine
A c
H CX II
ο II H3C kc/ C ^N H
HC- N ^ O Η (IV)
"NH
X I NH, Guanine
OH Ο — P — O - C H
// \ _ Ο ο
HC NH II I Uracil
OH ρ O- O ο
(III)
determine its properties. RNA, whether in RNA viruses or in healthy cells, is usually found as single strands, although in several viruses it exists as double-stranded RNA. Of course, DNA exists always as a double- stranded helix, except in single-stranded DNA viruses.
the biological
function of viral components
—coding
Although apparently each virus produces its own distinct protein coat, the only known function of the protein is to provide a protective sheath- ing for the nucleic acid of the virus. Protein itself has no infectivity, although its presence generally increases the infectivity of the nucleic acid. In inoculations with intact virus particles (virions) the protein does not seem to assist or to affect the nucleic acid either in its functions or its composition, since inoculations with nucleic acid alone can cause infec- tion and lead to synthesis of new nucleic acid and also of new protein, both being identical with those of the original virus. The synthesis, composition, and structure of the protein, on the other hand, depend entirely on the nucleic acid component which alone is responsible for the synthesis and assembly of both the RNA and the protein.
Infectivity of viruses in most cases is strictly the property of their nucleic acid, which in most plant viruses is RNA. Some viruses require and carry within them an RNA transcriptase enzyme in order to multiply and infect. The capability, however, of the viral RNA to reproduce both itself and its specific protein, indicates that the RNA carries the genetic determinants of the viral characteristics. The expression of each inherited characteristic depends on the sequence of nucleotides within a certain area (cistron) of the viral RNA which determines the sequence of amino acids in a particular protein, either structural or enzyme. This is called coding and seems to be identical in all living organisms and the viruses.
The code consists of coding units called codons. Each codon consists of three adjacent nucleotides and determines the position of a given amino acid.
The amount of RNA, then, contained in each virus indicates the approximate length of, and the number of nucleotides in, the viral RNA.
This in turn determines the number of codons in each RNA and, there- fore, the number of amino acids that can be coded for. Since the protein subunit of viruses contains relatively few amino acids (158 in TMV), the number of codons utilized for its synthesis is only a fraction of the total number of codons available (158 out of 2130 in TMV). The remaining codons are presumably involved in the synthesis of several other proteins, either structural or enzymes, and it is these proteins that are apparently responsible to a large extent for the diseased conditions produced in many virus infections of plants.
VIRUS INFECTION AND VIRUS SYNTHESIS 557
virus infection and virus synthesis
Plant viruses enter cells only through wounds made mechanically or by vectors, or by deposition into an ovule by an infected pollen grain.
The nucleic acid (RNA) of the virus is first freed from the protein coat.
It then induces formation by the cell of enzymes called RNA- polymerases (= RNA-synthetases = RNA-replicases). These enzymes, in the presence of the viral RNA acting as a template and of the nucleotides that compose RNA, produce additional, RNA. The first new RNA pro- duced is not the viral RNA but a strand that is a mirror image of the virus, and which, as it is formed, is temporarily connected to the viral strand (Fig. 205). Thus, the two form a double-stranded RNA that soon separates to produce the original virus RNA and the mirror image (-) strand, the latter then serving as a template for more virus (+ strand) RNA synthesis.
The replication of some single-stranded RNA viruses that have parts of their RNA in two or more virus particles, of some rhabdoviruses, and of some double-stranded RNA viruses differs considerably from the above. In viruses in which the different RNA segments are present within two or more virus particles, all or most of the particles must be present in the same cell for the virus to replicate and for infection to develop. In the single-stranded RNA rhabdoviruses the RNA is not infectious because it is the (-) strand. This RNA must be transcribed by the virus-carried enzyme called transciptase into a (+) strand RNA in the host and the latter RNA then replicates as above. In the double-stranded RNA isometric viruses, the RNA is segmented within the same virus, is noninfectious and depends for its replication in the host on a transciptase enzyme also carried within the virus.
As soon as new viral nucleic acid is produced it induces the host cell to produce the protein molecules that will be the protein subunits and that will form the protein coat of the virus. Apparently, only a part of the viral RNA strand is needed to participate in the formation of the viral protein.
Since each amino acid on the protein subunit molecule is "coded" by three nucleotides of the viral RNA, for TMV, whose RNA consists of 6400 nucleotides and its protein of 158 amino acids, only 474 nucleotides are required to code the arrangement of the amino acids in the protein subunit.
Protein synthesis in healthy cells depends on the presence of amino acids and the cooperation of ribosomes, messenger RNA, and transfer RNAs. Each transfer RNA is specific for one amino acid which it carries toward and along the messenger RNA. Messenger RNA, which is pro- duced in the nucleus and reflects part of the DNA code, determines the kind of protein that will be produced by coding the sequence in which the amino acids will be arranged. The ribosomes seem to travel along the messenger RNA and to provide the energy for the bonding of the prear- ranged amino acids to form the protein (Fig. 206).
For virus protein synthesis, the part of the viral RNA coding for the viral protein plays the role of messenger RNA. The virus utilizes the
FIGURE 206.
Schematic representation of the basic functions in a living cell.
VIRUS INFECTION AND VIRUS SYNTHESIS 559
FIGURE 207.
Sequence of events in virus infection and biosynthesis. CW = cell wall, R = ribosome, Ν = nucleus, η = nucleolus, Ρ = polyribosome (polysome), Pp = protein subunit, VP = viral particle, ι Ι Amin o acid, « 4 — Viral RNA replicase, eooocB Transfer RNA, •v/v/v. or VR Viral RNA.
amino acids, ribosomes, and transfer RNAs of the host, but it becomes its own blueprint (messenger RNA), and the protein formed is for exclu
sive use by the virus as a coat (Fig. 207) or other functions.
During virus synthesis, parts of its nucleic acid also become involved with synthesis of proteins other than the viral coat protein. Some of these proteins are enzymes, either of the kinds already present in the host cell or entirely new, which may activate or initiate in the cell chemical reactions that, in turn, may affect the physiological functions of the cell.
When new virus nucleic acid and virus protein subunits have been produced, the nucleic acid seems to organize the protein subunits around it, and the two are assembled together to form the complete virus parti
cle, the virion.
The site or sites of the cell in which virus RNA and protein are synthe
sized and in which these two components are assembled to produce the virions have not yet been determined with absolute certainty. Studies with TMV suggest that the virus RNA, after it is freed from the protein coat, moves into the nucleus and possibly the nucleolus, where it rep
licates itself. The new virus RNA is then released into the cytoplasm, where it serves as a messenger RNA and, in cooperation with the ribo-
somes and transfer RNAs, produces the virus protein subunits. The assembly of virions follows, also in the cytoplasm. In other viruses, the synthesis of viral nucleic acid and protein, as well as their assembly into virions, seems to take place in the nucleus, from which the virus particles are then released into the cytoplasm.
The first intact virions appear in plant cells approximately 10 hours after inoculation. The virus particles may exist singly or in groups and may form amorphous or crystalline inclusion bodies within the cell areas (cytoplasm, nucleus, nucleolus) in which they happen to be.
translocation and distribution of viruses in plants
For infection of a plant by a virus to take place, the virus must move from one cell to another and must multiply in most, if not all, cells into which it moves. In their movement from cell to cell, viruses follow the path- ways through the plasmodesmata connecting adjacent cells (Fig. 208).
Viruses, however, do not seem to move through parenchyma cells unless they infect the cells and multiply in them, thus resulting in continuous
Abrasive
Wounded cel l .Cuticle
pidermis Plasmodesmata|
Parenchyma Nucleus
Viral nuclei c aci d
Virus take n i n b y wounded cel l
Viral nuclei c aci d freed fro m coa t protein
Viral nuclei c aci d replicate s i n cell. Som e mov e t o adjacen t cells throug h plasmodesmata .
In phloem , vira l nuclei c aci d o r virus i s carrie d wit h th e photo - synthate throughou t th e plan t
Viral nuclei c aci d o r viru s reaches phloe m vesse l through plasmodesmat a of parenchym a cell s
Viral nuclei c aci d multiplie s i n ne w cell s an d spreads t o adjacen t cells . Som e o f th e earl y formed nuclei c aci d i s coate d wit h protei n and form s virus .
FIGURE 208.
Mechanical inoculation and early stages in the systemic distribution of viruses in plants.
SYMPTOMS CAUSED BY PLANT VIRUSES 561 and direct cell-to-cell invasion. In leaf parenchyma cells the virus moves
approximately 1 mm, or 8 to 10 cells, per day.
Although some viruses appear to be more or less restricted to cell-to- cell movement through parenchyma cells, a large number of viruses are known to be rapidly transported over long distances through the phloem.
Transport of viruses in the phloem apparently occurs in the sieve tubes, in which they can move as rapidly as 15 cm in the first 6 minutes.
However, most viruses require 2 to 5 or more days to move out of an inoculated leaf. Once the virus has entered the phloem, it moves rapidly in the phloem toward growing regions (apical meristems) or other regions of food utilization in the plant, such as tubers and rhizomes (Fig. 209). For example, when potato virus is introduced into the basal leaves of young potato plants, it moves rapidly up the stem, but when plants already forming tubers are similarly inoculated, the virus does not move upward for more than 30 days while it moves downward into the tubers. Once in the phloem, the virus spreads systemically throughout the plant and reenters the parenchyma cells adjacent to the phloem through plas- modesmata.
The distribution of viruses within plants varies with the virus and the plant. The development of local lesion symptoms has been considered as an indication of the localization of the virus within the lesion area (Fig.
210); although this is probably true in some cases, in several diseases the lesions continue to enlarge and, sometimes, development of systemic symptoms follows, indicating that the virus continued to spread beyond the borders of the lesions.
In systemic virus infections, some phloem-translocated viruses seem to be limited to this tissue and to a few adjacent parenchyma cells. These include such diseases as potato leaf roll, cereal yellow dwarf, etc. Viruses causing mosaic-type diseases are not generally tissue-restricted, although there may be different patterns of localization. Mosaic virus-infected plant cells have been estimated to contain between 100,000 and
10,000,000 virus particles per cell. Systemic distribution of some viruses is quite thorough and may involve all living cells of a plant. Other viruses, however, seem to leave segments or gaps of tissues that are virus free. Some viruses invade newly produced apical meristematic tissues almost immediately, while in other cases growing points of stems or roots of affected plants apparently remain free of virus.
symptoms caused by plant viruses
The most common and sometimes the only kind of symptoms produced is reduced growth rate of the plant, resulting in various degrees of dwarfing or stunting of the entire plant. Almost all viral diseases seem to cause some degree of reduction in total yield, and the length of life of virus- infected plants is usually shortened. These effects may be severe and easily noticeable, or they may be very slight and easily overlooked.
FIGURE 209.
Schematic representation of the direction and rate of translocation of a virus in a plant. (Adapted from G. Samuel (1934) Ann. Appl. Biol. 2 1 : 9 0 - 1 1 1 . )
The most obvious symptoms of virus-infected plants are usually those appearing on the foliage, but some viruses may cause striking symptoms on the stem, fruit, and roots, with or without symptom development on the leaves (Fig. 211 A, B). In almost all virus diseases of plants occurring in the field, the virus is present throughout the plant (systemic infection) and the symptoms produced are called systemic symptoms. In many plants inoculated artificially with certain viruses, and probably in some natural infections, the virus causes the formation of small, usually necro-
SYMPTOMS CAUSED BY PLANT VIRUSES 563
FIGURE 210.
Local lesions caused by two strains of a virus (tobacco ringspot) on mechanically inoculated leaves (cowpea).
tic lesions only at the points of entry (local infections), and the symptoms are called local lesions. Many viruses may infect certain hosts without ever causing development of visible symptoms on them. Such viruses are usually called latent viruses, and the hosts are called symptomless car- riers. In other cases, however, plants that usually develop symptoms upon infection with a certain virus may remain temporarily symptomless under certain environmental conditions (e.g., high or low temperature), and such symptoms are called masked. Finally, plants may show acute or severe symptoms soon after inoculation that may lead to death of the host; if the host survives the initial shock phase, the symptoms tend to become milder (chronic symptoms) in the subsequently developing parts of the plant, leading to partial or even total recovery. On the other hand, symptoms may progressively increase in severity and may result in gradual (slow) or quick decline of the plant.
The most common types of plant symptoms produced by systemic virus infections are mosaics and ringspots.
Mosaics, characterized by light-green, yellow, or white areas inter- mingled with the normal green of the leaves or fruit, or of whitish areas intermingled with areas of the normal color of flowers or fruit. Depending on the intensity or pattern of discolorations, mosaic-type symptoms may be described as mottling, streak, ring pattern, line pattern, veinclearing, veinbanding, chlorotic spotting, etc.
Ringspots, characterized by the appearance of chlorotic or necrotic rings on the leaves and sometimes also on the fruit and stem. In many ringspot diseases the symptoms, but not the virus, tend to disappear after onset and to reappear under certain environmental conditions.
A large number of other less common virus symptoms have been described (Fig. 211) and include stunt (e.g., tomato bushy stunt), dwarf
Cucumber mosai c o n
Tobacco mosai c sq u a sh Peppe r Cucumbe r
Pear rin g patter n M a i z e d w a r f m 0 S a i c Tuli p breakin g Tobacc o P ^ g f " * * El m ringspo t
Veinclearing vei n bandin g Vei n necrosi s Potat o lea f rol l Grape fa n lea f Tomat o shoestrin g (Cue. mosai c virus )
FIGURE 211.
Kinds of symptoms caused by viruses in plants.
(e.g., barley yellow dwarf), leaf roll (e.g., potato leaf roll), yellows (e.g., beet yellows), streak (e.g., tobacco streak), pox (e.g., plum pox), enation (e.g., pea enation mosaic), tumors (e.g., wound tumor), pitting of stem (e.g., apple stem pitting), pitting of fruit (e.g., pear stony pit), and flatten- ing and distortion of stem (e.g., apple flat limb). These symptoms may be accompanied by other symptoms on other parts of the same plant.
physiology of
virus-infected plants
Plant viruses do not contain any enzymes, toxins, or other substances considered to be involved in the pathogenicity of other types of patho- gens, and yet cause a variety of symptoms on the host. The viral nucleic
PHYSIOLOGY OF VIRUS-INFECTED PLANTS 565
Stunting B a n o n Q b u n c h y t op Citru s tristez a Coco a swo l le n shoo t ste m pitting
Apple f lo t lim b F ^ r r o u g h b a r k s t e m . Graf t brow n lin e ^ a n t e ^ El m zonot e conke r necrosis
Apple russe t ring Appl e sca r ski n Peo r ston y pi t Cucumber mosai c
Citrus wood y gal l Clove r woun d tumo r o n gladiolu s bul b
Tomato ringspo t Blackberr y Tomat o spotte d j o m n t n Potat o yello w Plu m po x o n aprico t
on grap e sterilit y wil t asperm y dwar f
FIGURE 211 (continued)
acid (RNA) seems to be the only determinant of disease, but the mere presence of RNA or complete virus in a plant, even in large quantities, does not seem to be sufficient reason for the disease syndrome, since some plants containing much higher concentrations of virus than others may show milder symptoms than the latter or they may even be symp- tomless carriers. This indicates that viral diseases of plants are not due primarily to depletion of nutrients that have been diverted toward syn- thesis of the virus itself, but to other more indirect effects of the virus on the metabolism of the host. These effects are brought about probably through the virus-induced synthesis of new proteins by the host, some of which are biologically active substances (enzymes, toxins, etc.) and can interfere with the normal metabolism of the host.
Viruses generally cause a decrease in photosynthesis through a de- crease in chlorophyll per leaf, in chlorophyll efficiency, and in leaf area per plant. Viruses usually cause a decrease in the amount of growth- regulating substances (hormones) in the plant, frequently by inducing an increase in growth-inhibiting substances. A decrease in soluble nitrogen during rapid virus synthesis is rather common in virus diseases of plants,
and in the mosaic diseases there is a chronic decrease in the levels of carbohydrates in the plant tissues.
Respiration of plants is generally increased immediately after infection with a virus, but after the initial increase the respiration of plants in- fected with some viruses remains higher, while with other viruses it becomes lower than that of healthy plants, and with still other viruses it may return to normal.
The amounts of nonvirus nitrogenous compounds in diseased plants seem to be generally lower than those found in healthy plants, probably because the virus, which in some virus-host systems may account for 33 to 65 percent of the total nitrogen in the plant, is formed at the expense of the normal levels of nitrogenous compounds in the plant. When the plant, however, is provided with high nitrogen nutrition, the amount of total nitrogen in diseased plants may be higher than that in healthy plants, especially after completion of the phase of rapid virus synthesis.
It appears, therefore, that many of the functional systems of the plant are directly or indirectly affected by virus infection. Certain degrees or types of such metabolic derangements can probably be tolerated by the plant and do not cause any symptoms, while others probably have a deleterious effect on the host and contribute to symptom development.
The effects of virus on nitrogenous compounds, on growth regulators, and on phenolics, have often been considered to be the immediate causes of various types of symptoms, since the first two are so profoundly involved in anything concerned with plant growth and differentiation, and since the oxidized products of phenolics may themselves, because of their toxicity, be responsible for the development of certain kinds of necrotic symptoms.
transmission
of plant viruses
Plant viruses rarely, if ever, come out of the plant spontaneously. For this reason, viruses are not disseminated as such by wind or water, and even when they are carried in plant sap or debris they generally do not cause infections unless they come in contact with the contents of a wounded living cell. Viruses, however, are transmitted from plant to plant in a number of ways such as vegetative propagation,- mechanically through sap; and by seed, pollen, insects, mites, nematodes, dodder, and fungi.
TRANSMISSION OF
VIRUSES BY VEGETATIVE PROPAGATION
Whenever plants are propagated vegetatively by budding or grafting, by cuttings, or by the use of tubers, corms, bulbs, or rhizomes, any viruses present in the mother plant from which these organs are taken will almost
TRANSMISSION OF PLANT VIRUSES 56 7
always be transmitted to the progeny (Fig. 212). Considering that almost all fruit and many ornamental trees and shrubs are propagated by bud- ding, grafting, or cuttings, and that many field crops, e.g., potatoes, and most florist's crops are propagated by tubers, corms, or cuttings, this means of transmission of viruses is the most important for all these types of crop plants. Transmission of viruses by vegetative propagation not only makes the new plants diseased, but in the cases of propagation by budding or grafting, the presence of a virus in the bud or graft may result in appreciable reduction of successful bud or graft unions with the rootstock and, therefore, in poor stands.
Transmission of viruses may also occur through natural root grafts of adjacent plants, particularly trees, the roots of which are often intermin- gled and in contact with each other. For several tree viruses, natural root grafts are the only known means of tree-to-tree spread of the virus within established orchards.
MECHANICAL TRANSMISSION OF VIRUSES THROUGH SAP
Mechanical transmission of plant viruses in nature by direct transfer of sap through contact of one plant with another is uncommon and rela- tively unimportant. Such transmission may take place between closely
By buddin g B y graftin g / 1
By cutting s
Through natura l roo t graft s Throug h dodde r FIGURE 212.
Transmission of viruses, mycoplasmalike organisms, and other pathogens by vegetative propagation, natural root grafts, and through dodder.
spaced plants following a strong wind that could cause the leaves of adjacent plants to rub together and, if wounded, to exchange some of their sap, and thus transmit any virus present in the sap (Figs. 213 and 214).
Potato virus X (PVX) seems to be one of the viruses most easily transmit- ted that way. When plants are wounded by man during cultural practices in the field or greenhouse and some of the virus-infected sap adhering to the tools, hands, or clothes is accidentally transferred to subsequently wounded plants, virus transmission through sap may be rapid and wide- spread and, as in the case of TMV on tobacco and tomato, may result in very serious losses. Virus-infected sap transferred from plant to plant on the mouthparts or body of animals feeding on and moving among the plants may on rare occasions lead to virus transmission.
The greatest importance of mechnical transmission of plant viruses stems from its indispensability in studying almost every facet of the viruses that cause plant diseases, since all investigations of virus outside the host are dependent on the ability to demonstrate and measure the infectiousness of the material.
For mechanical transmission of a virus from one plant to another, tissues of the infected plant believed to contain a high concentration of the virus, i.e., young leaves and flower petals, are ground with a mortar and pestle or with some other grinder (Fig. 213). Breakage of the cells
Virus-injected Youn g disease d Disease d leave s Leave s groun d i n Straine d infecte d Infecte d Infecte d sa p picke d u p plant leave s collecte d an d buffe r o r buffe r wit h pestl e sa p sa p o n fingers , gauz e pad ,
water place d i n glas s rod , brus h etc .
mortar
Cotyledons, primar y leaves , Infecte d sa p rubbe d o n health y Inoculate d plant s Inoculate d Loca l lesion s or regula r leave s ar e duste d plant s wit h fingers,gauz e pad , mus t i n som e case s plant s kep t i n
with abrasiv e powde r glas s rod , brush,etc . b e rinse d wit h greenhous e o r Symptom s develo p i n water immediatel y growt h chambe r 2 t o 21 day s
FIGURE 213.
Typical mechanical or sap transmission of plant viruses.
TRANSMISSION OF PLANT VIRUSES 569 results in release of the virus in the sap. Sometimes a buffer solution,
usually phosphate buffer, is added for stabilization of the virus. The expressed sap is then strained through cheesecloth and is centrifuged at low speeds to remove tissue fragments, or at alternate low and high speeds if concentration or purification of the virus is desired. The crude or partially purified sap is then applied to the surface of leaves of young plants which have been previously dusted with an abrasive such as 600-mesh Carborundum added to aid in wounding of the cells. Applica- tion of the sap is usually made by gently rubbing the leaves with a cheesecloth or gauze pad dipped in the sap, with the finger, a glass spatula, a painter's brush, or with a small sprayer. In successful inocula- tions, the virus enters the leaf cells through the wounds made by the abrasive or through broken leaf hairs and initiates new infections. In local-lesion hosts, symptoms usually appear within 3 to 7 or more days, and the number of local lesions is proportional to the concentration of the virus in the sap. In systemieally infected hosts, symptoms usually take 10 to 14 or more days to develop. Sometimes the same plants may first develop local lesions and then systemic symptoms. In mechanical trans- mission of viruses, the taxonomic relationship of the donor and receiving (indicator) plants is unimportant, since virus from one kind of plant, whether herbaceous or a tree, may be transmitted to dozens of unrelated herbaceous plants (vegetables, flowers, or weeds).
Although viruses are almost always transmitted by budding or graft- ing, several viruses, especially of woody plants, have not yet been trans- mitted mechnically. The possible reasons for this failure seem to be that some viruses are not present in high enough concentration in the donor plant, they are unstable in sap or are quickly inactivated by inhibitory substances released or formed upon grinding of the cells, and also because some viruses, e.g., those causing yellows-type diseases, apparently re- quire that they be introduced into specific tissues (phloem) if they are to cause infection.
SEED TRANSMISSION
About one hundred viruses have been reported to be transmitted by seed.
As a rule, however, only a small portion (1 to 30 percent) of the seeds derived from virus-infected plants transmit the virus, and the frequency varies with the host-virus combination (Fig. 214). In a few cases, e.g., tobacco ringspot virus in soybean, the virus may be transmitted by almost 100 percent of the seeds of infected plants, and in others, seed transmission may be quite high, e.g., 28 to 94 percent in squash mosaic virus in muskmelon, 50 to 100 percent in barley stripe mosaic virus in barley. Even within a species, however, different varieties or plants inocu- lated at different stages of their growth may vary in the percentages of their seeds that transmit the virus.
In most seed-transmitted viruses, the virus seems to come primarily from the ovule of infected plants, but several cases are known in which the virus in the seed seems to be just as often derived from the pollen that fertilized the flower.
Ο
σ
£ Disease d Health y Disease d Health y
*~ Throug h natura l lea f contac t an d rubbin g Throug h handlin g
·- Viru s infecte d tre e Flowe r o f virus - Viru s move s fro m polle n Viru s move s fro m Previousl y health y in bloo m infecte d tree . int o flowe r o f health y flowe r t o th e res t tre e no w infecte d
Virus i n polle n tre e o f th e tre e wit h th e viru s
FIGURE 214.
Virus transmission through direct contact, handling, seed, and pollen.
POLLEN TRANSMISSION
Virus transmitted by pollen may infect not only the seed and the seedling that will grow from it, but more important, it can spread through the fertilized flower and down into the mother plant, which thus becomes infected with the virus (Fig. 214). Such plant-to-plant transmission of virus through pollen is known to occur, for example, in stone fruit ringspot virus in sour cherry.
Although pollination of flowers with virus-infected pollen may result in considerably lower fruit set than is produced with virus-free pollen, transmission of pollen-carried virus from plant-to-plant is apparently quite rare or it occurs with only a few of the viruses.
INSECT TRANSMISSION
Undoubtedly the most common and economically most important means of transmission of viruses in the field is by insect vectors. Mem
bers of relatively few groups of insects, however, can transmit plant viruses (Fig. 215). The order Homoptera, which includes both aphids
TRANSMISSION OF PLANT VIRUSES
Aphid (wingless)
Aphid feedin g o n leaf
Aphid (winged)
Planthopper
Scale insec t Plan t bu g
Thrips Beetle Grasshopper
FIGURE 215.
Insect vectors of plant viruses. Insects in second row also transmit mycoplasmas and rickettsialike bacteria.
(Aphidae) and leafhoppers (Cicadellidae or Jassidae), contains by far the largest number and the most important insect vectors of plant viruses.
Certain species of several other families of the same order also transmit plant viruses, but neither their numbers nor their importance compare with the Aphidae and Cicadellidae. Among these families are the white flies (Aleurodidae), the mealy bugs and scale insects (Coccoidae), and the treehoppers (Membracidae). A few insect vectors of plant viruses belong to other orders, such as the true bugs (Hemiptera), the thrips (Thysanop- tera), the beetles (Coleoptera), and the grasshoppers (Orthoptera). The most important virus vectors, i.e., aphids, leafhoppers, and the other groups of H o m o p t e r a , as well as the true bugs, have piercing and sucking
m o u t h p a r t S ; all the other groups of insect vectors have chewing mouth- parts and virus transmission by the latter is much less common.
Insects with sucking mouthparts carry plant viruses on their stylets (style-borne or nonpersistent viruses) or they accumulate the virus in- ternally and, after passage of the virus through the insect tissues, they introduce the virus into plants again through their mouthparts (circula- tive or persistent viruses). Some circulative viruses may multiply in their respective vectors and are then called propagative viruses. Viruses trans-
571
mitted by insects with chewing mouthparts may also be circulative or they may be carried on the mouthparts.
Aphids are the most important insect vectors of plant viruses and transmit the great majority of all stylet-borne viruses (Fig. 216). As a rule several aphid species can transmit the same stylet-borne virus and the same aphid species can transmit several viruses, but in many cases the vec- tor-virus relationship is quite specific. Aphids generally acquire the stylet-borne virus after feeding on a diseased plant for only a few seconds (30 seconds or less) and can transmit the virus after transfer to and feeding on a healthy plant for a similarly short time of a few seconds. The length of time aphids remain viruliferous after acquisition of a stylet-borne virus varies from a few minutes to several hours, after which they can no longer transmit the virus. In the few cases of aphid transmission of circulative viruses, aphids cannot transmit the virus immediately but must wait several hours after the acquisition feeding, but once they start to transmit the virus, they continue to do so for many days following the removal of the insects from the virus source. In aphids transmitting stylet-borne viruses, the virus seems to be borne on the tips of the stylets, it is easily lost through the scouring that occurs during probing of host cells, and it does not persist through the molt or egg.
At least 10 plant viruses are transmitted by leafhoppers, including viruses with double-stranded RNA, bacilliform viruses, and small isomet- ric viruses.
Leafhopper-transmitted viruses cause disturbances in plants that arise primarily in the region of the phloem. All leafhopper-transmitted viruses are circulatory, several are known to multiply in the vector (propagative), and some persist through the molt and are transmitted to a greater or
FIGURE 216.
A winged aphid vector of a plant virus (barley yellow dwarf) sucking up juices, and possibly virus, from an oat stem. (Photo courtesy Dept. Plant Pathol., Cornell Univ.)
TRANSMISSION OF PLANT VIRUSES 573 lesser degree through the egg stage of the vector. Most leafhopper vectors
require a feeding period of one to several days before they become vir- uliferous, but once they have acquired the virus they may remain viruliferous for the rest of their lives. There is usually an incubation period of 1 to 2 weeks between the time a leafhopper acquires a virus and the time it can transmit it for the first time.
Mites of the family Eriophyidae have been shown to transmit nine vi- ruses, including agropyron mosaic, currant reversion, wheat streak mosaic, peach mosaic, and fig mosaic viruses. These mites have piercing and sucking mouthparts (Fig. 217). Virus transmission by eriophyid mites seems to be quite specific, since each of these mites has a restricted host range and is the only known vector for the virus or viruses it transmits.
Some of the mite-transmitted viruses are stylet borne, while others are circulatory and, of the latter, at least one persists through the molts.
In addition to the eriophyid mites, one mite of the family Tet- ranychidae (spider mites) has also been known to transmit a plant virus, potato virus Y.
Approximately one dozen plant viruses have been shown to be transmit- ted by one or more species of three genera of soil-inhabiting, ectoparasitic nematodes (Fig. 217). Nematodes of the genera Longidorus and MITE TRANSMISSION
NEMATODE TRANSMISSION
Eriophyid mit e
Virus transmissio n b y nematode s Spider mit e
Mite vector s o f plan t viruse s
\ . /
Plant infecte d wit h
virus an d fungu s Fungal zoosporangi a i n roo t
of virus-infecte d plan t Virus-carrying Zoospor e infect s zoospores leav e ne w plan t an d plant transmit s viru s
FIGURE 217.
Transmission of plant viruses by nematodes, mites, and fungi.
Xiphinema are vectors of polyhedral-shaped viruses such as tobacco ringspot, tomato ringspot, raspberry ringspot, tomato black ring, cherry leaf roll, brome mosaic, grape fanleaf, and other viruses, while nematodes of the genus Trichodorus transmit two rod-shaped viruses, tobacco rattle, and pea early browning viruses. Nematode vectors transmit viruses by feeding on roots of infected plants and then moving on to roots of healthy plants. Larvae as well as adult nematodes can acquire and transmit viruses, but the virus is not carried through the larval molts or through the eggs, and after molting, the larvae or the resulting adults must feed on a virus source before they can transmit again.
FUNGUS TRANSMISSION
The root-infecting fungus Olpidium transmits at least four plant viruses, tobacco necrosis, cucumber necrosis, lettuce big vein, and tobacco stunt viruses. Four other fungi, Synchytrium, Polymyxa, Spongospora, and Pythium, transmit respectively, potato virus X, wheat mosaic virus, potato mop top virus, and beet necrotic yellow vein virus, and pea false leaf roll. Some of these viruses apparently are borne internally in, and others on, the resting spores and the zoospores, which upon infection of new host plants introduce the virus and cause symptoms characteristic of the virus they transmit (Fig. 217).
DODDER TRANSMISSION
Several plant viruses can be transmitted from one plant to another through the bridge formed between the two plants by the twining stems of the parasitic plant dodder [Cuscuta sp.)(Fig. 212). A large number of viruses have been transmitted in this way, frequently between plants belonging to families widely separated taxonomically. The virus is usually transmitted passively in the food stream of the dodder plant, being ac- quired from the vascular bundles of the infected plant by the haustoria of dodder and, after translocation through the dodder phloem, it is intro- duced in the next plant by the new dodder haustoria produced in contact with the vascular bundles of the inoculated plant.
purification
of plant viruses
Isolation or, as it is usually called, purification of viruses is most com- monly obtained by ultracentrifugation of the plant sap. This involves 3 to 5 cycles of alternate high (40,000 to 100,000 g or more) and low (3000 to 10,000 g) speeds. Ultracentrifugation concentrates the virus and separates it from host cell contaminants. Several modifications of the ultracen- trifugation technique, particularly density-gradient centrifugation, are
SEROLOGY OF PLANT VIRUSES 575
Young leave s with symptom s
Diseased leave s ground i n buffe r in Warin g blende r
Leaf debri s Cheesecloth Sap wit h viru s Tissue homogenat e i s
strained throug h cheeseclot h Sap wit h viru s poure d int o centrifuge tube s an d i s spu n a t low spee d ( 3 - 1 0 , 0 0 0 g )
e = 3
Virusstill in sa p
Stirrin< ring roa
Pellet (discarded ) Supernatant wit h
virus i s collecte d Ultracentrifuge tube s fille d wit h supernatant an d place d i n fixed - angle roto r o f ultracentrifug e
Supernatant η is discarde d / /
1/
mTubes spu n a t hig h Viru s sediment s Viru s i n speed i n ultracentrifug e an d form s tin y pelle t i s ( 4 0 , 0 0 0 - I 5 0 , 0 0 0 g ) pelle t a t botto m resuspend -
oftube e d i n buffe r
Low an d highspee d centrifugation step s repeated 2- 3 times .
Virus i s kep t suspende d ~ * S S f l f c^ ^ S
in buffe r o r water , o r _ ^ ! ? Π P^pare d is purifie d furthe r b y J 7 tCbe s U Q
density gradien t Γ = Ί . ~ . . centrifugation. U J. 1 0 % l-2m l viru s
piF-20% suspensio n 1111—30% 1 ayere d o n W M -a t w sucros e
gradient
Tubes centrifuge d in swinging-bucke t rotors a t highspee d in ultracentrifug e
• p l p v i r u s mm ban d Virus particle s move togethe r as a ban d
Virus ban d collect ed a s sep e rate fraction throug h puncture i n botto m of tub e
Virus fractio n i s place d in cellulos e dialysi s tubing an d sucros e i s removed b y dialysi s i n buffer solutio n o r water
FIGURE 218.
Steps in the purification of plant viruses.
presently employed in virus purification with excellent results (Fig. 218).
In all these methods, the virus is finally obtained as a colorless pellet in a test tube and may be used for infections, electron microscopy and serology.
serology of plant viruses
When a virus protein or any other foreign protein (antigen) is injected into a mammal (rabbit, mouse, horse), or bird (chicken, turkey), it results in the appearance of substances (antibodies) in the blood serum which react specifically with the antigen injected. The virus and its antibody are brought together in several ways, the most common being the precipitin reaction. In this, the antibodies and antigens are mixed in solution (pre
cipitin test), or they meet at the interface between two solutions contain
ing each separately (ring interface test), or they diffuse toward each other through an agar gel and meet in a zone in suitable concentrations (Ouchterlony test). Sometimes the antigen is absorbed on the surface of a large particle such as a cell or plastid and these are precipitated by addition of antibodies (agglutination reaction). In all these tests the reac-
A.IMMUNIZATION, COLLECTIO N on d PREPARATIO N O F ANTISERU M
Purified antige n (Ag) , (virus, bacteria,myco - plasmas,etc.) wit h o r without adjuvan t i s taken u p i n syringe .
=T3
&2
Antigen i s injecte d once o r mor e i n muscl e (thigh) o r vei n (ear ) o f animal
Several week s o r month s later,blood i s obtaine d from ea r o r hear t o f injected animal . Bloo d is allowe d t o clo t over night
Clotted bloo d i s centrifuged a t 5,00 0 rpm fo r lOmin . Clea r antiserum (supernatant ) seperates fro m bloo d cells (pellet ) Pelle t i s discarded
The antiseru m (Ab )
T o
(serum plu s antibodies ) is poure d int o smal l vials. Glyceri n i s usuall y added an d th e whol e is kep t frozen . B.SEROLOGICAL TEST S (A g i s dilute d wit h appropriat e bufferiA b i s dilute d wit h physiologica l salin e (0S5%NaCI ) i n wate r o r buffer )
I Rin g Interfac e Tes t
110 1:2 0 W O 1=8 0 | : 16 0 et c Antigen Udilutions)
|~Hnterface (reaction
area) Antiserum!
(constant dilution)
Visible reactio n (cloud y area)forms a t interfac e within minute s o r hour s after mixin g homologou s Ag an d Ab .
2.Microprecipitin Tes t
A g d l u t i p nA b c o n t r o |
Plastic Petr i dis h A dro p o f eac h A g dilution i s adde d per bo x o f eac h column _rid mad e wit h wax penci l
ύ
A dro p o f eac h - A b dilutio n i s
! adde d pe r bo x
! o f eac h ro w
ι ί π π α π ο
• • • • • •
The tw o drop s ar e stirred togethe r i n each bo x
Cloudy precipitat e
\ form s i n drop s wit h
\proper dilution s o f
\m\&\ \nomologou s A g an d Ab (withi n hours )
3.0uchterlonv Double-Diffusio n Tes t
^ r ^ r ^ Plasti c Petr i dis h wit h Λ \ 0 . 9 % agaros e o r / \ lonaga r No . 2 i n % { \ buffe r plu s 0 . 2 %
L J sodiu m azid e
Holes ar e punched i n agar ge l wit h cork borer s
Agar plu g i s removed fro m well wit h pipe t connected t o vacuum
Antigen (Ag ) i s place d in middl e wel l an d different antiser a (Ab) i n th e periphera l wells o r vice-vers a
Ag an d A b diffuse i n ge l in al l direction s and towar d each othe r
' Wher e th e diffusion patterns o f homologous Ag an d A b meet.a whit e band forms . Ab, A U X
Reaction o f unknow n antigen wit h know n antisera identifie s the antige n
FIGURE 219.
Production of antisera and serological tests for identification of unknown pathogens.
tion of antigen and antibody becomes visible either by precipitation of the two on the bottom of the test tube or by formation of a band at the interface where the two meet (Fig. 219).
The uses of plant virus serology are numerous. Thus, it is used to determine relationships between viruses, to identify a virus causing a plant disease, to detect virus in foundation stocks of plants, and to detect symptomless virus infections. It can also be used to measure virus quan
titatively, to locate the virus within a cell or tissue, to detect plant viruses in insects, and to purify a virus.
nomenclature and classification of plant viruses
Naming of plant viruses usually has been based on the most conspicuous symptom they cause on the first host they have been studied in. Thus, a virus causing a mosaic on tobacco is called tobacco mosaic virus, while
IDENTIFICATION OF PLANT VIRUSES 57 7
the disease itself is called tobacco mosaic; another virus causing ringspot symptoms on tomato is called tomato ringspot virus, and the disease is called tomato ringspot, and so forth. Considering, however, the vari- ability of symptoms caused by the same virus on the same host plant under different environmental conditions, by different strains of a virus on the same host, or by the same virus on different hosts, it becomes apparent that this system of nomenclature leaves much to be desired.
All viruses belong to the kingdom VIRA. Within the kingdom there are two virus divisions, DNA viruses and RNA viruses, depending on whether the nucleic acid of the virus is DNA or RNA. Viruses within each division are either helical or cubical (polyhedral). Within each sub- division there may be viruses possessing one or two strands of RNA or DNA, possessing or lacking a membrane around the protein coat, con- taining or lacking certain substances, having certain symmetry of helix in the helical viruses or number of subunits in the cubical (polyhedral) viruses, size of the virus, and, finally, any other physical, chemical, or biological properties.
In many plant diseases assumed to be caused by viruses, no virus has yet been observed and it is possible that some of these diseases will be proven later to be caused by pathogens other than viruses or by as yet uncharacterized viruses. For those plant diseases, however, proven to be caused by viruses, a system of nomenclature and classification has been proposed (Fig. 220), in which the viruses are grouped according to the above-listed criteria, according to size and several additional criteria unique to plant viruses. The groups are named after a typical virus in the group and are accompanied by a cryptogram indicating whether the virus contains RNA (R) or DNA(D), single (1) or double stranded (2); the percent RNA or DNA and the molecular weight of the virus; whether the virus and its nucleocapsid are elongated (E) or spherical (S); and the kind of host(s) (S
= plants) and of vector, if any.
identification of plant viruses
Once the cause of a disease has been established as a virus, a series of tests may be necessary to determine its identity. The host range of the virus, i.e., the hosts on which the virus induces symptoms and the kinds of symptoms produced, may help to differentiate this virus from several others. Transmission studies should indicate whether the virus is trans- mitted mechanically and to what hosts, or by insects and which insects, and so on; each new property discovered helping to further characterize the virus. If the virus is transmitted mechnically, certain properties of the virus such as its thermal inactivation point, i.e., the temperature required for complete inactivation of the virus in untreated crude juice during a 10-minute exposure, its longevity in vitro, its dilution end point, i.e., the highest dilution of the juice at which the virus can still cause infection, may be used to narrow the possibilities to just a few viruses. If, at this stage, the identity of the virus is suspected, serological tests may be used and if they are positive, a tentative identification may be made. Examina- tion of the virus in the electron microscope, and inoculation of certain
